1. Field of the Invention
The present invention relates to an A/D converting device, an A/D converting method and an optical pulse testing apparatus, which are suitable for an optical test.
2. Description of the Related Art
The OTDR (Optical Time Domain Reflectometry) method for detecting a breaking point of an optical cable and for measuring a loss or the like, which is caused by a connecting point thereof has been developed. In the OTDR method, the breaking point of the optical cable is detected and the loss or the like, which is caused by the connecting point of the optical cable is measured by measuring a pulse height of a light returned back to the incident end of the optical cable.
An optical pulse testing apparatus using the OTDR method has been used. The optical pulse testing apparatus comprises a pulse generator, a connector, a fiber to be measured, a coupler, an optical fiber, an APD (Avalanche Photo Diode), an OP-amp (operational amplifier), an A/D (Analog to Digital) converter, a signal processing part or the like.
In the optical pulse testing apparatus, the optical pulse is generated by the pulse generator to transmit it to the coupler. The optical pulse passes through the coupler to transmit it to the optical fiber. While the optical pulse transmitted to the optical fiber from the coupler passes through the optical fiber, (that is, in proportion to the length of the optical fiber from the incident end thereof,) the loss which the optical fiber has causes the power of the optical pulse to be damped.
Because backward scattering lights are caused by the characteristics of the optical fiber at each point of the optical fiber, the backward scattering lights are transmitted to the APD through the coupler and the optical fiber. The backward scattering lights passing through the optical fiber and received by the APD are converted into electric current waveform signals corresponding to the intensities of the received lights by the APD to output them to the OP-amp. The electric current waveform signals outputted from the APD are amplified by a predetermined gain with the OP-amp and are converted into the voltage waveform signals. Then, the voltage waveform signals are converted into digital signals having a predetermined number of bits, which are sampled by a predetermined sampling frequency with the A/D converter, to output the digital signals to the CPU circuit in the signal processing unit. In the CPU circuit, the level of the backward scattering light is identified on the basis of the digital signal outputted from the A/D converter to display the waveform of the backward scattering light received by the APD or that of the Fresnel reflection light received by the APD on the display.
In the A/D converter used in the optical pulse testing apparatus, because the frequency band width of the voltage waveform signal inputted into the A/D converter is several hundreds MHz, in order to convert the inputted voltage waveform signal into the digital signal having a low distortion by a predetermined resolution (number of bits), the sampling frequency having not less than several hundreds MHz and the resolution enabling the change of the voltage to be detected more particularly are required.
When it is attempted that these requirements are satisfied by using one A/D converter, the cost of the one A/D converter is higher than that of the A/D converter for covering several tens MHz band, which is used in signal processing systems, such as broadcasting media or the like. For this reason, according to an earlier development, when the A/D converter for covering several tens MHz band is used, the following method is applied. Each sampling timing is changed in each sweeping operation in order to reconfigure the bit data obtained by carrying out the sweeping operations a plurality of times. Thereby, the same digital data having a desirable resolution can be obtained as the data to be obtained by using the expensive A/D converter in one sweeping operation. As a result, the A/D converter for covering several hundreds MHz band can be realized by a low cost.
However, it is necessary that the sweeping operations are carried out a plurality of times to obtain the desirable resolution by using the above method. It is thought that the following method is carried out. A plurality of A/D converters are assigned to each sampling timing. As a result, the digital signal having a desirable resolution can be obtained by carrying out only one sweeping operation.
In the earlier optical pulse testing apparatus, when the method in which a plurality of A/D converters are used is carried out, the following problems are caused.
FIG. 7A shows an output characteristic of an A/D converter 21. FIG. 7B shows an output characteristic of an A/D converter 22. When the A/D converters 21 and 22 convert the same analog signal into a digital signal, the output level of the digital signal into which the analog signal is converted by the A/D converter 21 is different from that of the digital signal into which the analog signal is converted by the A/D converter 22 in the vicinity of the reference level shown by the dashed line, as shown in FIGS. 7A and 7B. FIG. 8 shows a circuit construction of an A/D converting unit using the A/D converters 21 and 22 in the optical pulse testing apparatus.
In FIG. 8, each digital signal outputted from the A/D converters 21 and 22 is outputted to a CPU circuit 24 through a switch 23. The switch 23 is controlled by a switching signal outputted from the CPU circuit 24 so that the digital signal outputted from the A/D converter 21 and the digital signal outputted from the A/D converter 22 are outputted to the CPU circuit 24 alternately.
In case that the digital signal outputted from the A/D converter 21 is outputted to the CPU circuit 24 through the switch 23 at the first sampling timing T1, the digital signal outputted from the A/D converter 22 is outputted to the CPU circuit 24 through the switch 23 at the next sampling timing T2.
In the A/D converting unit, when the voltage waveform signal of which the level is reduced with the lapse of time as shown in FIG. 9A is inputted into the A/D converters 21 and 22, there is a problem that the digital signal into which the voltage waveform signal is converted has noises which do not correspond to the reduction of the input level of the voltage waveform signal.
Because of the difference between the output characteristic of the A/D converter 21 and that of the A/D converter 22, which are shown in FIGS. 7A and 7B, even though the input level of the voltage waveform signal inputted into the A/D converter 21 is the same as that of the A/D converter 22, the output level of the digital signal outputted from the A/D converter 21 is different from that of the A/D converter 22. As a result, when the two digital signals into which the voltage waveform signal is converted by the A/D converters 21 and 22 are outputted alternately, the output level is not constant in the vicinity of the reference level as shown in FIG. 9B.
In case of using a plurality of A/D converters, because of quantization errors caused by an irregularity of a gain of the A/D converter and that of linearity of the converted digital signal, the output value of the digital signal into which the A/D converter 21 converts the analog signal is not coincident with that of the A/D converter 22, even though the analog signal inputted into the A/D converter 21 is the same as that of the A/D converter 22. The problem that the waveform measurement value is not constant in each sampling when the two A/D converters 21 and 22 are used is caused by the above non-coincidence of the output values. In particular, when the data to be analyzed by a bit resolution which is much higher than that of the A/D converter is processed by using an A/D convert with dither method like an optical pulse testing apparatus, the above non-coincidence of the measurement values causes serious problems.
In order to solve the above-described problems, an object of the present invention is to provide an A/D converting device, an A/D converting method and an optical pulse testing apparatus, which carry out the control to switch one digital signal outputted from one A/D converter to another properly so that the dispersion of output characteristic of each A/D converter can be cancelled while a plurality of A/D converters are used.
That is, in accordance with one aspect of the present invention, the A/D converting device for converting an input signal having a cyclic waveform into a digital data by using a plurality of A/D converters connected in parallel with each other, comprises;
a selecting circuit for selecting each digital data from those of the plurality of A/D converters in an order, and
an output order changing circuit for changing the order in which each digital data is selected from those of the plurality of A/D converters by the selecting circuit.
According to the present invention, when digital data is obtained at one sampling point, digital data outputs of a plurality of A/D converters are synthesized. In case that the digital data outputted from a plurality of A/D converters are synthesized, the digital data output having fewer errors can be obtained according to the central limit theorem, as compared with the case in which one A/D converter is used.
The selecting circuit may select each digital data to be outputted from each A/D converter from those of the plurality of A/D converters in the order by synchronizing with a sampling timing of the each A/D converter, and the output order changing circuit may change the order by synchronizing with the sampling timing of the each A/D converter when each digital data is selected from those of the plurality of A/D converters by the selecting circuit.
In case that the digital data outputted from a plurality of A/D converters at each sampling point of the time series cyclic signal are synthesized, the cyclic noise having a repeated pattern, which is caused at the sampling timing of each A/D converter by assigning each A/D converter to each sampling point of the time series cyclic signal, can be removed.
In accordance with another aspect of the present invention, the A/D converting method for converting an input signal having a cyclic waveform into a digital data by using a plurality of A/D converters connected in parallel with each other, comprises the steps of;
selecting each digital data from those of the plurality of A/D converters in an order, and
changing the order in which each digital data is selected from those of the plurality of A/D converters by carrying out the selecting step.
According to the present invention, when digital data is obtained at one sampling point, digital data outputs of a plurality of A/D converters are synthesized. In case that the digital data outputted from a plurality of A/D converters are synthesized, the digital data output having fewer errors can be obtained according to the central limit theorem, as compared with the case in which one A/D converter is used.
The selecting step may be carried out by selecting each digital data to be outputted from each A/D converter from those of the plurality of A/D converters in the order by synchronizing with a sampling timing of the each A/D converter, and the changing step may be carried out by changing the order by synchronizing with the sampling timing of the each A/D converter when each digital data is selected from those of the plurality of A/D converters by carrying out the selecting step.
In case that the digital data outputted from a plurality of A/D converters at each sampling point of the time series cyclic signal are synthesized, the cyclic noise having a repeated pattern, which is caused at the sampling timing of each A/D converter by assigning each A/D converter to each sampling point of the time series cyclic signal, can be removed.
In accordance with another aspect of the present invention, the optical pulse testing apparatus for converting an optical signal into a digital data by using a plurality of A/D converters connected in parallel with each other to output an integrated value of each digital data outputted from each A/D converter as a measuring result, comprises;
a selecting circuit for selecting each digital data from those of the plurality of A/D converters in an order, and
an output order changing circuit for changing the order in which each digital data is selected from those of the plurality of A/D converters by the selecting circuit.
According to the present invention, when digital data is obtained at one sampling point, digital data outputs of a plurality of A/D converters are synthesized. In case that the digital data outputted from a plurality of A/D converters are synthesized, the digital data output having fewer errors can be obtained according to the central limit theorem, as compared with the case in which one A/D converter is used.
The selecting circuit may select each digital data to be outputted from each A/D converter from those of the plurality of A/D converters in the order by synchronizing with a sampling timing of the each A/D converter, and the output order changing circuit may change the order by synchronizing with the sampling timing of the each A/D converter when each digital data is selected from those of the plurality of A/D converters by the selecting circuit.
In case that the digital data outputted from a plurality of A/D converters at each sampling point of the time series cyclic signal are synthesized, the cyclic noise having a repeated pattern, which is caused at the sampling timing of each A/D converter by assigning each A/D converter to each sampling point of the time series cyclic signal, can be removed.
In accordance with another aspect of the present invention, the A/D converting device comprises;
a plurality of A/D converters connected in parallel with each other, for carrying out a converting operation a plurality of numbers of times, in which an analog signal is converted into a digital data,
a selecting circuit for selecting each A/D converter from the plurality of A/D converters at each sampling timing during the converting operation to sample the converted digital data,
a selecting circuit controlling unit for controlling the selecting circuit so that when one A/D converter is selected at one sampling timing during the Nth converting operation, another A/D converter is selected at the one sampling timing during the (N+1)th converting operation,
an adding circuit for integrating the digital data sampled at the one sampling timing during each converting operation, and
a processing unit for processing the integrated digital data.
In accordance with another aspect of the present invention, the A/D converting method comprises the steps of;
carrying out a converting operation a plurality of numbers of times, in which an analog signal is converted into a digital data by using a plurality of A/D converters connected in parallel with each other,
selecting each A/D converter from the plurality of A/D converters at each sampling timing during the converting operation to sample the converted digital data,
selecting one A/D converter at one sampling timing during the Nth converting operation when another A/D converter was selected at the one sampling timing during the (Nxe2x88x921)th converting operation,
integrating the digital data sampled at the one sampling timing during each converting operation, and
processing the integrated digital data.
In accordance with another aspect of the present invention, the optical pulse testing apparatus comprises;
a plurality of A/D converters connected in parallel with each other, for carrying out a converting operation a plurality of numbers of times, in which an optical pulse is converted into a digital data,
a selecting circuit for selecting each A/D converter from the plurality of A/D converters at each sampling timing during the converting operation to sample the converted digital data,
a selecting circuit controlling unit for controlling the selecting circuit so that when one A/D converter is selected at one sampling timing during the Nth converting operation, another A/D converter is selected at the one sampling timing during the (N+1)th converting operation,
an adding circuit for integrating the digital data sampled at the one sampling timing during each converting operation, and
a processing unit for processing the integrated digital data.